I float on my thermarest

The answers so far surprise me.  This needs a poll.  Swimming and cliff diving (read on before you flame me...actually cliff jumping feet first and only after carefully checking water depth) is a favorite every trip especally after a hot day (and even when not hot) or a long day sitting in the canoe fishing.  It is absolutely refreshing.  Risks?  I suppose I could drown, but I could drown at the local pool, or wading/fishing in the local river, or in two inches of water in a bucket for that matter, but I haven't in 47 years, so I guess I will assume that risk.  I could cut my foot.  That has happened.  You need to be careful and so far it hasn't been anything a good first aid kit couldn't take care of.  My vote?  Last one in is a rotten egg and who needs a bathing suit?!!

The BW and Q are great places to swim!  I've swum alone on solo trips, but in the shallows or with my PFD.  Sometimes at the end of a long, hard, hot portage I just wade in and fall into the water for relief.  After sweating that much it's not like I'm getting "wetter".
One of our regular group of campers spends almost all her free camp time exploring the shoreline from a mink's point of view.  Swimming along and checking things out on shore when they interest her.
Plusses are: clean water, few weeds, nothing in the water to worry about.  We wear shoes so we can get in/out on slippery rocks without tearing our toe nails off!  If you do swim in weeds in mid-summer you might get an itchy rash from water fleas that live in the weedy areas - but nothing that will bother you much.   

I responded as a birthday suit swimmer but in reality my swimming is a mix of birthday suit, shorts, and/or PFD depending on the situation.

I sometimes do as IC does at the end of hot, sticky portages.  I barely slow down as I shed the pack and just keep right on walking into the water.  Usually I remember that I have a knife and other things in my pockets that I don't want to get wet before I get too far, though.  If the need is dire, I walk back to dry land, shed the shorts (but not the BVDs since this is a portage) and head back in for a full immersion.  Other times I will stop short and just stick my head into the water...an amazingly effective cool-down without the total immersion.

In camp, if I'm swimming to get clean, it's b-day suit time.  If I'm swimming to just escape the heat, I usually wear shorts and will at least have the PFD handy in case I start feeling lazy.

Cliff-jumping?  Been there, done that, and will do it again if the opportunity arises.  It's not a must-do activity or anything, but if it's convenient and safe (and yes, safe is a matter of personal opinion) then it's just another way to enjoy the trip.

Swimming usually becomes bathing as well, especially if I'm in an especially small tent in august!  I tend to swim in the birthday suit unless the situation requires more formal attire